Electric motors are used to convert electrical energy into mechanical energy. Permanent magnet field motors have low horsepower (hp) ratings and are used commercially for servo applications such as machine tools, robotics, and high-performance computer peripherals. Of particular interest are higher hp levels required in heavy industry and commercial production lines. For example, polyphase AC induction motors, primarily three-phase, are used to drive fans, pumps, and material handling devices. These motors operate at low frequencies, less than 2 kHz and typically at 60 Hz.
FIGS. 1a and 1b illustrate a 3-phase induction motor 10 that converts electrical energy into mechanical energy. As an aside, an induction motor can be operated as a generator which converts mechanical energy into electrical energy. The motor 10 includes a 3-phase stator winding 12 and three separate coils 14a, 14b, and 14c that are wound physically around a stator form 16 at 120 degree intervals and encapsulated in an insulator 18. Each coil includes a large number of wire strands 20 that are provided individually with an insulative sheath. The stator winding 12 is mounted inside a motor housing 22 and does not move. A rotor 24 is wound or provides means for a closed coil 26 and is mounted on a set of bearings 28 inside the stator winding 12. A drive shaft 30 extends axially from rotor 24 through motor housing 22.
Power delivered by an electric company to industrial plants for large motors is typically three-phase and is connected to drive coils 14a, 14b, and 14c. At low frequencies, the current flows through inductance coils of the motor 10 such that electrical energy is converted into mechanical energy. Specifically, the induced current flowing through the inductance coils produces a rotating magnetic field that cuts across rotor coil 26 inducing a circulating current in coil 26 that develops its own magnetic field in the rotor 24. Field intensifiers 32 are positioned around the interior of stator winding 12 to amplify its rotating magnetic field. The interaction between the rotating magnetic field and the rotor field produces the motor action that rotates shaft 30. Sinewave type motors drive the shaft at the line frequency, typically 60 Hz. Inverter type motors convert the fixed frequency line voltage into a controllable-frequency drive voltage so that the shaft can be rotated at different frequencies, typically less than 2 kHz.
In heavy industry and commercial production lines, unexpected motor failure disrupts the line, which wastes time and money. Further, this oftentimes results in discarded products, and may damage other online systems. The cost of reconditioning or replacing a motor is negligible compared to the expense associated with an unscheduled shut down. Consequently, commercial production lines routinely are shut down on a fixed maintenance schedule to test the motors and determine whether a component failure has occurred.
A 1985 Electric Power Research Institute (EPRI) study of failure modes in three phase induction motors revealed that 41% of the motors failed because of the rotor bearings, 37% failed due to problems associated with the stator winding insulation, 10% failed from rotor problems and 12% failed for a variety of other reasons. Although the dominant failure mode is bearing failure, stator winding insulation failure is the most significant from a user's perspective because its the most unpredictable. A stator winding failure may cause the performance of the motor to degrade or to fail entirely.
A 1985 brochure "Failure in Three-Phase Stator Windings" from Electrical Apparatus Service Association, Inc. (EASA) illustrated the typical winding failures in three-phase stators when exposed to unfavorable operating conditions--electrical, mechanical or environmental. Typical winding failures include a single-phase failure in which one phase of the winding is opened, phase-to-phase shorts, turn-to-turn shorts, winding grounded to the slot (intensifier), and thermal deterioration of insulation. Single-phase failures are usually caused by a blown fuse, an open contractor, broken power line or bad connections. The phase-to-phase, turn-to-turn and grounded winding failures result from contaminants, abrasion, vibration or voltage surges. Thermal deterioration is caused by imbalanced voltages, load demands exceeding the rating of the motor, locked rotor condition and power surges.
Current test procedures are conducted off line and coarsely measure degradation in the performance of the motor in its low frequency operating range, below 2 kHz, to detect the existence or non-existence of one of these failure modes in the stator winding. Based upon his or her experience, a technician decides whether a failure has occurred and what action to take. The risk is that the motor will fail or severely degrade before it is pulled off the line or that perfectly good motors will be rejected mistakenly. Known test procedures neither detect the onset of the damage that eventually causes one of these stator winding failures, identify the failure mechanism responsible for the damage, determine the susceptibility of the windings to further damage, nor predict when failure will occur.
Typically, motors are subjected to partial discharge and surge tests to detect a stator winding failure. In the partial discharge test, a technician discharges a capacitor across the stator winding and observes the time domain voltage response to detect spikes on the 60 Hz envelope. The magnitude of the spikes is a rough indicator of stator winding damage. In the surge test, a technician applies a large voltage pulse to each phase of the winding and compares their time domain current responses to detect asymmetry that is indicative of damage. At best these tests detect whether a failure has occurred, neither is sensitive enough to detect the onset and progression of damage to the winding prior to an actual failure.